Semiconductor devices, such as thyristors, have long been used as switches in high current electrical circuits to open or close a current conducting path between a source of current and an electrical load under control of associated operating circuitry which provides the appropriate command signals to the control or gate input terminal of the semiconductor device. A resistance welding circuit is one typical example of such a high current electrical load and the familiar resistance welder is one type of device in which such semiconductor device has been employed.
The ability of a particular semiconductor to conduct electrical current is not unlimited. The current or power handling capability of the semiconductor is limited by the phenomena of self-heating: when the temperature of the substrate or chip from which the semiconductor is fabricated becomes excessive, the semiconductor is permanently destroyed. To prevent such failures at present limiters and controls are provided in the associated electrical circuitry, particularly in those applications where the semiconductors are expensive, over a hundred dollars apiece, and are thus not easily replaced. Those existing limiters and controls employ either open loop design heat sinks, or slow response time closed loop limiters, which are known to those skilled in the art.
Both techniques are critically sensitive to overload induced thermal self-destruction. Common closed loop semiconductor temperature limiters employ bi-metallic temperature sensitive switches, thermistors and other electrical or mechanical devices mounted externally on the pack or case, which contains the semiconductor chip, to sense over-temperature. The resulting signals sensed are included in a feedback circuit to exert a control function to inhibit further self-heating operation of the semiconductor.
An obvious disadvantage of such existing technique is that the thermal sensors are physically remote from the substrate or chip in which the heating occurs and the heat must be transferred from the semiconductor chip through the case to the sensor. The limited thermal conductivity of the heat transfer path and the thermal "lag" or delay in detecting the heating limits the ability of those protection circuits to respond in a timely manner to save the semiconductor from destruction. Effectively the design of existing overheat limiting circuitry to handle fault conditions becomes a compromise between cost and complexity and the fault level at which the semiconductor is permitted to be destroyed.